Apparatus and method for fabricating a deflection mirror tower

ABSTRACT

A deflection mirror tower (DMT) of a multiple-disk array, optical storage system includes a plurality of prism members, each having an angular mirror surface, arranged along a vertical axis of the DMT and configured at a predetermined angular orientation to deflect a laser beam to a respective disk surface. The DMT is generally fabricated using a two-level partitioning process. Several prepared glass substrates, i.e., one for each mirror type, are initially sliced into bars having predetermined geometries. A bar from each substrate is sequentially aligned to a master reference and bonded to adjacent bars to form an intermediate bar stack. The stack is then segmented into a plurality of DMT components, each having a plurality of prism members.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of co-pending U.S. patent application Ser. No.08/303,895, filed on Aug. 16, 1994, entitled "APPARATUS AND METHOD FORFABRICATING A DEFLECTION MIRROR TOWER" which is a continuation of Ser.No. 07/847,455 filed Mar. 6, 1992, now abandoned.

FIELD OF THE INVENTION

This invention relates generally to optical systems and, morespecifically, to a deflection mirror tower for an optical storagedevice.

BACKGROUND OF THE INVENTION

Conventional optical storage devices typically employ a single opticaldisk having a single recording surface for storing information. Use of asingle disk allows the optical components, e.g., a mirror, to bearranged relative to the recording surface in a manner that optimizesthe size and cost of the device. Although this results in a low-costdevice having a relatively small form factor, its storage capacity islimited to that provided by a single surface. Copending and commonlyassigned U.S. patent application of Lee et al., for OPTICAL STORAGESYSTEM, filed herewith, describes a multiple-surface system in which thebeam from a single laser is steered by a stationary galvanometer-rotatedmirror to optical heads associated with the respective recordingsurfaces. The heads are mounted on a carriage that-moves them radiallyover the surfaces for access to selected data tracks on the surfaces.

Specifically, the rotating mirror selectively directs the beam to one ofa vertical array of uniquely oriented deflection mirrors. When adeflection mirror receives the beam, it reflects it along a planeparallel to and close to a corresponding disk surface. The beam thenpasses through an imaging lens on the way to a 45° mirror that redirectsthe beam radially toward an objective lens in the optical headassociated with that surface. The objective lens, in turn, converges thebeam on a small spot on the selected data track.

Therefore, it is apparent that the deflection mirrors must be preciselypositioned. Moreover, the mirror array must be small to providecompatibility with a form factor for the overall system that iscomparable with that of conventional multiple-disk magnetic disk drives.Further, the array must be manufactured within the cost constraints ofprior single-disk optical components.

Several fabrication techniques might be employed to produce a deflectionmirror array. One approach involves the construction of a precasthousing and bonding of the individual mirrors thereto. Each mirrorelement must be individual handled because of the accuracy required inassembling the mirrors to the housing. This is a substantially difficultand time consuming process because of the small size of the mirrors. Theassembly process may further necessitate manual alignment of the mirrorarray component relative to the remaining optical components of thesystem. Such individualized construction and alignment procedures aretime consuming and costly.

An alternative approach involves plastic molding of the entire array.Although this process provides a low-cost component, current plasticmolding technology produces mirror surfaces having a degree of flatnessthat is insufficient for the intended use of the mirrors. Moreover,because the mirrors are oriented at different angles with respect to thelaser beam, each mirror requires a different dielectric coating toreflect the beam. It is difficult and time consuming to provide suchcoatings in an assembly of mirrors.

Therefore, it is desirable to provide a deflection mirror tower having aplurality of mirrors precisely arrayed along a contoured surface thereoffor use in a multiple-disk optical storage device.

It is also desirable to provide a method for mass producing deflectionmirror towers, which, if manufactured using existing processes, may beimpractical and too costly.

In addition, it is desirable to provide a fabrication process thatenables installation of mass produced deflection mirror towers in astorage drive without further adjustment of the mirrors.

SUMMARY OF THE INVENTION

Briefly, a deflection mirror tower (DMT) of a multiple-disk array,optical storage system comprises a plurality of prism members arrangedalong a vertical axis of the DMT, the axis being parallel to therotating axis of the disks. Each prism member has a mirror surfacefacing along the optical path to the disk array and having apredetermined angular orientation to deflect an optical beam to arespective disk surface.

In accordance with the invention, the DMT is fabricated using atwo-level partitioning process. The process begins with the preparationof a set of glass substrates in the form of thick, rectangular plates.One surface of each plate is made optically smooth, e.g., by grinding. Areflective dielectric coating is then applied to that surface.Ultimately, each of the plates will be cut into mirrors whose angle ofreflection will be the same, but will be different from that of themirrors cut from each of the other plates. Accordingly, the reflectivecoating differs among the respective plates according to the angles ofreflection of the mirrors to be cut therefrom.

Next, each of the plates are sliced into bars whose top surfaces are thereflective surfaces. In each plate, the angle of the slices relative tothe reflective surfaces depends on the angle of reflection of themirrors to be made from the plate. A set of bars, one from each plate,is then stacked to form a block with the reflective surfaces of the barsforming a continuous, segmented surface of the block. Specifically, theslice surfaces of adjacent bars are bonded together and the angles ofthe slices relative to the mirror surfaces of the bars provide coarsecontrol of the orientations of the mirror surfaces relative to eachother. Fine adjustment of the orientations, as needed, can easily beprovided by shims before the bars are bonded together. Finally, theblock is sliced transversely to the bars to provide a plurality ofDMT's.

It will be apparent that the overall process provides for precise, yetlow-cost, fabrication of the DMT's. For example, large mirror surfacesare prepared and coated, so that the cost of preparing each mirror isminimal. Moreover, the preparation and handling of the relatively largebars to provide precise mirror orientations is also a relativelylow-cost procedure, whose cost is divided among a number of DMT's.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

FIG. 1 is a diagram of an optical beam distribution system for amultiple-disk, magneto-optical storage device in which the method andapparatus of the present invention may be utilized;

FIG. 2 is a perspective view of a deflection mirror tower (DMT) inaccordance with the invention;

FIG. 3A is a schematic diagram of a base substrate used to fabricate theDMT of FIG. 2;

FIG. 3B is a schematic diagram of the substrate of FIG. 3A sliced into aplurality of "positive" bars having a predetermined thickness andangular orientation;

FIG. 3C is a schematic diagram of a tapered positive bar having a pairof shallow capillary grooves formed along a side thereof;

FIG. 4 is a diagrammatic view of a "negative" master assembly used as areference to facilitate alignment and proper angular orientation of thepositive bars during fabrication of the DMT in accordance with theinvention;

FIG. 5 is a perspective view of a fixture used to fabricate the masterassembly of FIG. 4;

FIG. 6A is a diagrammatic view of the master assembly in mating relationto the positive bars during fabrication of DMT;

FIG. 6B is a perspective view of an embodiment of a datum referencefixture used during the fabrication of the DMT;

FIG. 6C is a perspective view, partially broken away, of an alternateembodiment of the fixture of FIG. 6B; and

FIG. 7 is a perspective view of a positive bar stack sliced intoindividual DMT units in accordance with the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 depicts an optical beam distribution system 10 for amultiple-disk array, magneto-optical storage device. The beamdistribution system 10 includes a stationary optics package 12 forgenerating a laser beam and a galvanometer-controlled mirror, i.e.,"galvo mirror" 14, for distributing the beam to one of several opticalhead assemblies 15, each of which is associated with a recording surfaceof an optical disk 13. The galvo mirror 14 distributes the beam to adeflection mirror tower (DMT) 20 having a multi-faceted mirror surface22 arranged to deflect the beam so that it is passed to a head assembly15 of a selected disk. Specifically, each facet 22a of the surface 22 ispositioned at the same height as a corresponding head assembly 15 and itis oriented so as to reflect the beam from the galvo mirror 14horizontally toward a lens/mirror set 16,17 contained in a lens/mirrortower (LMT) 18. The lens/mirror set 16,17 redirects the beam to thecorresponding head assembly 15.

Referring also to FIG. 2, a DMT 20 is shown comprising a plurality ofdiscrete prism members 24 featured together as a block. Each member 24has a surface that provides one of the mirror facets 22a. In anexemplary embodiment of the invention, the storage device includes sixoptical disks with ten recording disk surfaces, each having anassociated optical head assembly 15. The distance between headassemblies 15 associated with opposite surfaces of a disk 13 isapproximately 8.0 millimeters (mm), while the distance between headassemblies of opposing surfaces of adjacent disks is approximately 6.0mm. As will be described herein, each member 24 of the DMT 20 has apredetermined thickness; however, only a specific portion of the mirrorfacet 22a of each member is used, depending upon the position of thecorresponding head assembly 15 within the disk array and height of thecorresponding imaging lens 16.

For example, as shown in FIG. 2, the imaging lenses 16₅ and 16₆ areclose together and the lenses 16₄ and 16₅ are relatively far apart.Thus, the lower portions of the mirror facets 22a₄ and 22a₆ and theupper portion of the facet 22a₅, which are opposite these lenses, areinvolved in beam reflection. On the other hand, the upper portions ofthe mirror facets 22a₄ and 22a₆ and the lower portion of the facet 22a₅,which are not opposite any lenses, are "dead space". The dead space onthe facets 22a₂ and 22a₃ accommodates a 45° relay mirror 30 thatreflects the beam from the optics package 12 to the galvo mirror 14.

As shown in FIGS. 1 and 2, the DMT 20 is configured such that thedistance the laser beam travels from the optics package 12 to an imaginglens 16 of the LMT 18 is substantially the same regardless of whichmirror facet 22a reflects the beam. Accordingly, the lengths L of themembers 24 progressively increase from the bottom of the DMT 20 to thetop to provide a generally arcuate contour mirror 22.

As will be described further herein, each member 24 is sliced from abase substrate having a flat mirror surface that is polished and coatedwith an appropriate reflecting coating. Angular slicing of the substrateresults in each member having a generally linear, yet obliquely orientedmirror surface; further, the orientation of each mirror surface isdependent upon its location on the DMT. When assembled vertically andparallel to the rotating axis of the optical disks, each member is anintegral part of the multi-faceted, generally arcuate mirror 22 of theDMT 20 configured to deflect the optical laser beam to a preciselocation on a respective disk surface. The illustrated DMT 20 includesten mirror facets 22a positioned at non-complex angles to ensure thatthe beam travels orthogonally to its respective disk surface.

Each DMT 20 is preferably formed from a plurality of base substrates,one for each recording disk surface of the optical drive. FIG. 3A is aschematic diagram of a generally long, rectangular substrate 32, whichis preferably made of a glass material, such as boro silicate crownglass. As noted, the long glass substrate provides a significantmotivation for the invention because it is easy to handle, especiallywhen preparing the mirror surface, as follows. Specifically, the topsurface 32a of the substrate 32 is "polished" to a smooth surface usinga conventional polishing machine. The surface 32a is then coated with anappropriate multi-layer dielectric reflecting material, such as titaniumdioxide (TiO₂) and silicon dioxide (SiO₂), using a conventionalevaporation process. Selection of the appropriate reflecting coatingdepends upon a number of criteria, including (i) the wavelength of thelaser beam; (ii) the angle of incidence of the beam with respect to eachmirror of the DMT; (iii) the desired reflectivity; and (iv) maximumphase shift related to polarization.

Each substrate 32 is then sliced lengthwise into a plurality of"positive" bars 34, each having a mirror surface 22a and having athickness t and angular orientation, as shown in FIG. 3B. As will beseen, the bars 34 are the foundation of the DMT component. Slicing ispreferably accomplished with a conventional ultrasonic cutter or diamondwheel cutter. The bars 34 are then preferably shaved to a slightlytapered configuration (FIG. 3C) and a pair of shallow capillary grooves36 are formed lengthwise along one side of each bar 34 to facilitatebonding, as described below. The end pieces 37 and 38 of the substrate32 are discarded. The bars 34 originating from a specific substrate 32are substantially similar in shape and dimension; however, the barsoriginating from different substrates have generally differentdimensions.

FIG. 4 depicts a "negative" master assembly 40 for use as a reference tofacilitate alignment and proper angular orientation of the positive bars34 when assembling a DMT 20. The mating surface 42 of the masterassembly 40 is precisely shaped with a contour that conforms to thedesired contour of the DMT mirror surface 22. Specifically, negativemaster bars 44 are preferably sliced from a glass substrate (not shown)in a tapered arrangement similar to formation of the positive bars 34.The master substrate has a slightly different glass composition fromthat of its positive counterpart, so that the master 40 has a differentthermal coefficient of expansion.

The negative bars 44 are preferably calibrated and aligned using afixture 80 depicted in FIG. 5. The fixture 80 is a generally U-shapedmember having sides 82a,b and a bottom surface 84. A plurality of holes86 located in side 82a and bottom 84 (not shown) accommodate screws 88used to adjust the bars to a proper position, as described below.

Specifically, a first negative bar 44a inserted into the fixture 80,with the surface 42a (FIG. 4) facing upwardly. The bar 44a is fixed inplace against the side 82b and aligned with an autocollimator (notshown) that is used to calibrate the remaining negative bars.Calibration is generally performed with the autocollimator by generatinga laser beam, directing it to the surface 42a of the bar 44a and thenanalyzing the reflected beam. Comparison of the generated beam andreflected beam indicates the angle of incidence at the surface 42a.Because of their tapered configuration, negative bars 44 may bephysically adjusted by the screws 88 to present a desired angle at thesurface 42a.

A second negative bar 44b is then inserted into the fixture 80, adjacentto the first bar 44a with their surfaces 42a disposed in frictionalcontact relation. The second bar 44b is thereafter calibrated, adjusted(if necessary) and bonded to the first bar 44a with an optical adhesive,such as ultra-violet (UV) cured glue. The calibration and bondingprocedure is repeated with each bar 44 until the master assembly 40 iscompleted.

As noted and referring to FIG. 6A, the master assembly 40 facilitatesalignment and proper angular orientation of the DMT mirror surface 22(FIG. 2); this is accomplished by disposing each positive bar mirrorfacet 22a in contact with a respective facet 42a of the master matingsurface 42. Specifically, a "color contact" technique is employed toensure proper alignment between the bars 34 and master assembly 40. Inaddition, the contact technique facilitates separation of the bars afterbonding. Color contact generally involves disposing a small amount ofliquid, such as alcohol or water, on the mirror facet 22a of eachpositive bar 34 prior to mating contact with the master 40. Thereafter,the facet 22a is joined in frictional contact relation to a respectivemating facet 42a of a negative bar 44, the mating facet 42a having asimilar, but opposite, angular orientation to that of the mirror facet22a. This provides substantial conformance of the mirror facet 22a tothe counter-angular mating surface orientation of the master assembly40. Moreover, color contact enables very tight tolerances on angles ofeach of the positive bars 34 since conformance with the master may bemonitored by observation of resultant color fringes.

Operationally, each positive bar 34 is inserted into a mating fixtureconfigured to enclose the master assembly 40 and subsequently is affixedto a respective bar 44 of the master. FIG. 6B illustrates an embodimentof a datum reference fixture 50 used during the DMT fabrication process.The fixture 50 encompasses the master 40 so that the mirror facet 22a ofeach positive bar 34 may be secured in color contact to the mating facet42a of a respective bar 44. The fixture 50 is configured to align thepositive bars 34 to the master 40 by limiting lateral displacement ofthe bars 34. This process also minimizes scratching of the negativemaster 40.

Specifically, each positive bar 34 stacked in vertical alignment in thedatum reference fixture 50, which is a generally U-shaped memberassembly having sides 52a,b preferably configured as ladders with aplurality of reference steps 54a,b. Each step 54a is substantiallyidentically-positioned in opposing, facing relation to a respective step54b. The steps 54a,b function as datum points, hereinafter generallydesignated 55, and the generally planar bottom surface 56 of the fixture50 functions as a datum surface. In a preferred embodiment, the sides52a,b are generally equal in length 1 to the secured bars and thefixture 50 has a back surface (not shown) that limits the rearwarddisplacement of the bars. In the illustrated example, there are fourdatum points (four steps on each side) for the fixture 50, with eachdatum configured to provide a predetermined arrangement of the positivebars 34 in relation to the spacing of the disks of the disk array, asdescribed below.

A first positive bar 34a is inserted into the fixture 50, with thesurface 22a (FIG. 4) facing inwardly. The bar 34a is placed upon thedatum surface 56 and is subsequently affixed to the respective bar 44.Because of its tapered configuration, the positive bar 34a may bephysically adjusted to present the desired mating contact at the surface42a. In accordance with the teachings of the invention, the extendedlength of the bars 34 reduce their sensitivity during the adjustment andalignment process.

A second positive bar 34b is then inserted into the fixture 50, adjacentto the first bar 34a with their surfaces 22a disposed in frictionalcontact relation. Here, datum points 55a are not used because theintersection between the first and second bars corresponds to therelatively close spacing (2.94 mm) between head assemblies 16 (FIG. 1)of adjacent disk surfaces. Because the datum points 55a are notutilized, the second bar 34b has a narrower width relative to the firstbar 34a to avoid contact with the projections 55a. The second bar 34b isthereafter adjusted (if necessary) and clipped to the first bar 34ausing clips 58.

A third positive bar 34c is then inserted into the fixture 50 and placedupon the first datum points 55a. The distance between the datum surface56 and first datum points 55a corresponds to the relatively greaterspacing (7.86 mm) between head assemblies 15 on either side of a disk.It can therefore be appreciated that the remaining interfaces betweenthe third (34c) and fourth (34d) bars, the fifth (34e) and sixth (34f),the seventh (34g) and eighth (34h), and the ninth (34i) and tenth (34j)bars 34 are not aligned to respective datum points 55b, 55c, 55d,whereas the fifth (34e), seventh (34g) and ninth (34i) bars do utilizethe respective datums 55b, 55c, 55d. The adjustment procedure isrepeated with each bar 34 until each positive bar is aligned withrespective bars 44 of the master assembly 40.

Once aligned, the positive bars 34 are bonded to each other by anadhesive, such as UV glue. The adhesive is preferably applied by aconventional capillary/vacuum process, which entails placing the loadedbar fixture 50, including the master 40, into a vacuum chamber, pouringthe glue into the capillary grooves 36 formed along the sides of thebars 34 and then releasing the vacuum to suck the glue into the grooves.Application of UV light subsequently cures the glue.

An alternate embodiment of the datum reference fixture is shown in FIG.6C. Here, each positive bar 34 is inserted into a mating fixture 60configured to enclose the master assembly 40 and subsequently is affixedto a respective bar 44 of the master. It can be seen that the matingfixture 60 is substantially similar to the datum reference fixture 50;however, the fixture 60 is arranged to encompass the master 40 such thatthe mirror facet 22a of each positive bar 34 may be lowered over andsecured in color contact to the mating facet 42a of a respective bar 44.The fixture 60 is configured to align the positive bars 34 to the master40 by limiting lateral displacement of the bars 34. This process alsominimizes scratching of the negative master 40. Repetition of thisprocedure for each bar type results in the creation of a "positive" barstack 70 once the aligned bars 34 are bonded to each other.

Because of the different thermal coefficients of expansion of theirmaterials, the positive bar stack 70 and master assembly 40 may beimmersed into hot water (approximately 70° F.) to facilitate separation;thereafter, they may be easily separated by applying pressure to theirinterface (snapping). The fixture 60 may then be removed and thepositive bar stack 70 is sliced into individual DMT units 20, asdepicted in FIG. 7, using an ultrasonic cutter or conventional diamondwheel cutter.

The 45° relay mirror 30 is then affixed to the DMT unit 20 preferably ata location defined by the dead space on the facets 22a ₂ and 22a ₃ (FIG.2). The relay mirrors are formed by slicing a rectangular glasssubstrate into a plurality of generally triangular substrates. The sidesof each substrate are then coated with a generally reflective material.The resulting relay mirror 30 is bonded to the DMT 20 using a3-dimensional fixture (not shown) adapted to accurately position themirror upon the arcuate surface 22 of the DMT. Accordingly, theinvention provides a high-precision, deflection mirror tower capable ofhigh-volume, repeatable production at low-cost.

The foregoing description has been directed to specific embodiments ofthis invention. It will be apparent, however, that variations andmodifications may be made to the described embodiments, with theattainment of some or all of their advantages. Therefore, it is theobject of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

What is claimed is:
 1. A method of fabricating a deflection mirror towerarrangement for an optical storage device, said tower arrangement havinga multi-faceted arcuate mirror surface configured to deflect an opticalbeam to one of a plurality of closely-spaced optical disks havingrecording disk surfaces configured to store data, said method comprisingthe steps of:constructing a plurality of positive bars, each having adefined angular orientation at a mirror surface thereof; constructing anegative master assembly having a plurality of negative bars, eachhaving a surface arranged to provide a facet of a multi-faceted matingsurface with a contour similar, but opposite, to said arcuate mirrorsurface of said deflection mirror tower; disposing said mirror surfaceof each positive bar in frictional color contact relation to arespective facet of said mating surface of said negative master, suchthat said positive bars are aligned adjacent to one another, saidrespective facet having a similar, but opposite, angular orientation tothat of said mirror surface of each of said positive bars; and creatinga plurality of prism assembly segments, each having an arcuate surfacecontour composed of said mirror surfaces of said positive bars, wherebyeach of said mirror surfaces provide a facet of said multi-facetedarcuate mirror surface of said tower arrangement to deflect said opticalbeam to any of said plurality of closely-spaced optical disks.
 2. Themethod of claim 2 wherein the step of constructing said plurality ofpositive bars comprises the steps of:providing a plurality of generallyrectangular substrates, one for each of said recording disk surfaces,each of said substrates having said mirror surface; coating said mirrorsurfaces with a reflecting material characterized by the wavelength ofthe optical beam, the angle of incidence of the beam with respect toeach mirror facet, the desired reflectivity coefficient, and the maximumphase shift related to polarization of said beam; and slicing eachsubstrate at a predetermined thickness and angle to create saidplurality of positive bars, such that the angular orientation of saidmirror surfaces of said bars are similar among said positive barsoriginating from the same substrate and are different among saidpositive bars originating from different substrates.
 3. The method ofclaim 2 wherein said reflecting material is a multi-layer dielectricreflecting material.
 4. The method of claim 3 wherein said multi-layerdielectric material is titanium dioxide and silicon dioxide.
 5. Themethod of claim 2 wherein the step of creating said plurality of prismassembly segments comprises the steps of:bonding said positive bars oneto another to create a prism assembly having an arcuate surface contour;separating said bonded prism bar assembly from said negative masterassembly; and slicing said bonded prism bar assembly into a plurality ofprism assembly segments, each having a plurality of positive bars.
 6. Adeflection mirror tower arrangement for an optical storage device, saidtower having a multi-faceted arcuate mirror surface configured todeflect an optical beam to one of a plurality of closely-spaced opticaldisks having recording disk surfaces configured to store data, saidtower arrangement comprising:means for constructing a plurality ofpositive bars, each having a defined angular orientation at a mirrorsurface thereof; means for constructing a negative master assemblyhaving a plurality of negative bars, each having a surface arranged toprovide a facet of a multi-faceted mating surface with a contoursimilar, but opposite to said arcuate mirror surface of said deflectionmirror tower; means for disposing said mirror surface of each positivebar in frictional color contact relation to the facet of a respectivenegative bar of said negative master, such that said positive bars arealigned adjacent to one another, said facet of the respective negativebar having a similar, but opposite, angular orientation to that of saidmirror surface of each of said positive bars, the means for disposingcomprises a datum reference fixture configured to align said positivebars to the facet of the respective negative bars of the master assemblyby limiting lateral displacement of said positive bars; and means forcreating a plurality of prism assembly segments, each having an arcuatesurface contour composed of said mirror surfaces of said positive bars,whereby each of said mirror surfaces provide a facet of saidmulti-faceted arcuate mirror surface of said tower arrangement todeflect said optical beam to any of said plurality of closely-spacedoptical disks.
 7. The deflection mirror tower arrangement of claim 6wherein said means for constructing said negative master assemblycomprises a fixture configured to align said negative bars forcalibration and bonding.
 8. The deflection mirror tower arrangement ofclaim 7 wherein said fixture has a generally U-shaped structure with aplurality of sides and a bottom surface and wherein said sides andbottom surface have a plurality of holes to accommodate screws foradjusting said negative bars to an aligned position.
 9. The deflectionmirror tower arrangement of claim 6 wherein said datum reference fixtureis a generally U-shaped assembly having a bottom surface and furtherhaving first and second sides configured as ladders with a plurality ofreference steps.
 10. The deflection mirror tower arrangement claim 9wherein each of said reference steps of said first side is substantiallyidentically-positioned in opposing, facing relation to a respective stepof said second side.
 11. The deflection mirror tower arrangement claim10 wherein each of said steps function as a datum point and wherein saidbottom surface functions as a datum surface.